This is extremely frustrating, and has been an issue for the past month. I cannot play Civilization V over the internet with any of my friends. Whenever I try to play a game with my girlfriend, it says that she disconnected whenever she tries to respond to me in chat (in the setup area before the game is started). According to her, on her side it says that I disconnected at that same moment. At this point, we try re-inviting each other to that same game (via steam). The person who was invited gets a message saying (error joining multiplayer). This happens no matter who is hosting the game.
I was successfully able to play several games in the public lobby with no problems at all. So, we tried both joining a public multiplayer game that someone else was hosting in the lobby. We were successfully able to join the game in the lobby, but just like before, everyone in the game disconnected one by one--as if us trying to play together was some kind of infectious plague. Afterwards my girlfriend gave up, and I just ended up going forward with that game without her. It worked just fine at that point--I was able to enter the game with them. At this point, it seems like there may be something wrong on my girlfriend's side. We tried putting down her firewall, turning off her VPN, and shutting down her Minecraft server, but nothing helped. We have tried this at least 100 times, allocating about 30 minutes of monotonously and painstakingly trying over and over again before we give up.
Finally, today we tried playing a three player game with a third friend of mine. The exact same thing happened. No matter which one of us hosted the game, we all get disconnected after about 30 seconds of accepting the invitation. Still suspecting that it might be a problem with my girlfriend's computer, me and my friend tried doing just a two player game without my girlfriend. We got the exact same thing: we both disconnect after about 30 seconds in the setup room. So that seems to rule out that it was something wrong on my girlfriend's side. So they both tried to play a two player game without me, and still got the same thing. So that rules out that there was something wrong on my side.
Can anyone shed some light on why I can't play any multiplayer games, unless it's with complete strangers?